Hitherto, in reducing NOx from the exhaust gas of a diesel engine, a method has been adopted in which the exhaust gas temperature is increased to the range of 300 to 400.degree. C., ammonia and urea each having the same number of moles as that of NOx are added to the exhaust gas, and after the addition, the NOx is brought into contact with a vanadium/titania catalyst and a zeolite catalyst so as to be reduced and decomposed to N.sub.2 and H.sub.2 O. This method, however, requires an additional preparation of ammonia or urea, and may cause secondary environmental pollution if too much ammonia or urea is added. Thus, to solve such problems, a method has been developed in which the hydrocarbons in the exhaust gas are forcibly enriched, and then the NOx in the exhaust gas is reduced and removed by transition metal-supporting metallosilicate catalysts including a copper/zeolite catalyst, or by transition metal-supporting alumina series composite oxide catalysts including a copper/alumina catalyst.
As methods of enriching the hydrocarbons in the exhaust gas, a method of adding a fuel into a suction pipe of an engine (Japanese Unexamined Patent Publication No. 4-358715), a method in which the timing of the main fuel injection is shifted and a small amount of the fuel is separately injected into the engine cylinders (Japanese Unexamined Patent Publication No. 3-253713), and a method of directly adding fuel or a special hydrocarbon into an exhaust pipe are included. In addition, as techniques for removing NOx by adding a hydrocarbon, which becomes a reducing agent, into the exhaust pipe, a method in which the hydrocarbon-added exhaust gas is guided to a catalyst bed so as to be reduced (Japanese Unexamined Patent Publication No. 4-358716), and a method in which a reducing agent hydrocarbon-added exhaust gas is cooled and guided to the catalyst bed (Japanese Unexamined Patent Publication No. 5-44445), etc., have been known.
However, in the case of a catalytic reaction in which NOx is cleaned by using a hydrocarbon as a reducing agent, the catalyst operating temperature is limited to the range of approximately 300 to 500.degree. C., although it varies with the type of catalyst and reducing agent. Particularly, if the catalyst and the reducing agent hydrocarbon used are limited to one type, respectively, the temperature range is further narrowed.
Thus, to extend the operating temperature range of the catalyst, various methods have been devised. One of them is a method, as disclosed in Japanese Unexamined Patent Publication No. 6-146870, in which a plurality of catalysts having different operating temperature ranges are disposed in series, and a cooling gas is injected over the catalysts so as to control the exhaust gas temperature to be in a range of high reduction efficiency. However, this exhaust gas cleaning device is not practical, because it encounters problems in that the volume of the catalyst and the pressure loss increase. In addition, a method (Japanese Unexamined Patent Publication No. 6-108825) has been known in which a hydrocarbon, which is a fuel, is used as a reducing agent, and is previously decomposed and modified by a cracking catalyst, etc., and then added to the exhaust gas. This method, however, encounters problems of bad decomposition performance of the cracking catalyst, and of bad follow-up properties to variations in the engine load. Further, a method of cooling and controlling the exhaust gas temperature using a heat exchanger (the above-described Japanese Unexamined Patent Publication No. 5-44445) is not practical because it encounters a problem in that soot adheres to a cooling water pipe to block the exhaust pipe, and requires frequent cleaning.
As described above, in cleaning NOx under an extension of the catalyst operating temperature range, i.e., under a wide-range of engine operating conditions, there are many outstanding problems.